Metallica peaked with Master Of Puppets and finally embraced commercial stuffs on their following album finally releasing a promotional video (One). The Black Album ideally would have been their last. That said I really don't begrudge them carrying on (Carry On Metallica stylee). I'd say my favourite track is definitely from Master Of Puppets and is probably the quite unique Orion.

http://www.youtube.com/watch?v=KfAtqCTs39A